Open Ocean: the Blue Desert

Common Thresher Shark

Sporting a resplendent, scythe-like upper lobe of the caudal fin — as
long or longer than its body — the Common Thresher (Alopias vulpinus) is
among the most instantly-recognizable of sharks. The function of this
hyperextended appendage has evoked much fanciful speculation and scientific
debate. The matter is not completely settled, but there are a few
observations that offer intriguing clues as to how this amazing structure
might be used.

Dramatic features, such as the caudal fin of thresher sharks, often seem
to cry out for dramatic explanations. So it is perhaps unsurprising that
reports of Threshers using their tails to hit or stun prey are persistent in
the literature. In winter 1865 an Irish ichthyologist claimed he actually
saw a Common Thresher rise up to the surface of Dublin Bay, use the tip of
its caudal fin to swat a wounded diver (probably Gavia immer, the Great
Northern Diver, known in North America as the Common Loon), and then swallow
it. Despite the ichthyologist’s otherwise solid reputation and well-known
stand on the need to back up ichthyological theory with observation in the
wild, some authorities doubt his report. In 1927, two prominent American
zoologists stated that the Thresher Shark’s tail is not sufficiently rigid
or muscular to strike an efficient blow.

Misgivings about the Irish report notwithstanding, it has long been
assumed that the Common Thresher uses the upper lobe of its long, slender
caudal fin to herd its prey. Known prey of the Thresher consists mostly of
small schooling fishes, such as mackerels (family Scombridae), Bluefish (Pomatomus
saltatrix), herrings (Clupeidae), needlefishes (Belonidae), and
lanternfishes (Myctophidae), but it also takes large, solitary fishes such
as lancetfishes (Alepisauridae) as well as squids, pelagic octopuses and
crustaceans. In the vastness of the open ocean, small schooling fishes and
squids would certainly be easier to catch if concentrated somehow. But a
prominent French shark biologist has seriously questioned the traditional
assumption that the Thresher uses its tail as a food-gathering device. He
points out that, when faced with a predator, the natural reaction of most
schooling or herding animals is to bunch closer together; thus, a Thresher
Shark could do just as well concentrating prey with an ordinary caudal fin
and has no need to act like an aquatic sheepdog.

There are too many reports of Thresher Sharks herding and using their
caudal fins to strike prey to discount them all. Some of these reports are
buttressed by compelling circumstantial evidence. One Russian ichthyologist
reported that 97% of all threshers caught on pelagic longlines in the
northwestern Indian Ocean were foul-hooked through the upper lobe of the
caudal fin, suggesting that the sharks were snagged as they tried to stun
hooked fishes. There are also field observations of Thresher Sharks using
their hyperextended caudal fins to capture fish prey, reported in the
scientific literature by credible witnesses. In a 1915 paper, a
well-respected American shark-watcher reported a most remarkable
observation. In shallow waters of the bight of Cape Lookout, North Carolina,
he actually saw a Common Thresher using its caudal fin to toss fish into its
mouth; one fish the shark had failed to seize was thrown a “considerable
distance”. Another account, this one by an oceanographer, seems an
unimpeachable report of a Thresher Shark using its caudal fin to stun prey.

Early one morning, while collecting plankton off the pier at Scripps
Institution of Oceanography (near La Jolla, California), the oceanographer
heard a nearby splash. At a distance of about 100 feet (30 metres), he saw a
swirl in the water like that made by a sea lion. A moment later, a
three-foot- (one-metre-) long, slender, flattened tail flashed above the
surface and lashed about “like a coachwhip”. A few minutes later he glimpsed
a shark coming toward the surface, swimming rapidly roughly 50 feet (15
metres) from the end of the pier. Almost immediately, the oceanographer saw
a small fish —which he thought might have been a California Smelt (Atherinopsis
californiensis) — swimming frantically just in front of the shark. A moment
later, the pursuing shark — now clearly identifiable as a six-foot (2-metre)
Thresher Shark — passed partly (about half-way) ahead of its prey, turned
quickly, and gave the same coach-whip lash with its caudal fin that the
oceanographer had seen earlier. The whip-stroke was repeated immediately,
“with very confusing speed” and it became evident that the victim was
seriously injured. At this point, however, the fish was almost under the
drip from the oceanographer’s net, which apparently frightened the Thresher
into darting away. The injured fish sank, swimming feebly, then came to the
surface and lay on its side for a while, until eventually it sank out of
sight. Although the oceanographer did not actually see the Thresher Shark
eat the fish it had injured, the speed and skill with which the Thresher
struck its prey and the accuracy demonstrated in its lashes at a single,
fast-moving target greatly impressed him. Anyone who has ever tried catching
goldfish bare-handed or picking up objects with his toes cannot help but be
impressed also.

If the Thresher’s caudal fin is capable of at least some of the amazing
feats with which it has been credited, it must add considerably to the
shark’s overall propulsive power. Although when hooked it does not perform
repeated acrobatic leaps like the Shortfin Mako (Isurus oxyrinchus), the
Common Thresher is ranked as a game fish by the International Game Fish
Association (IGFA), putting up a strong, determined fight and even leaping
on occasion. Some of these leaps are quite spectacular. One specimen was
recently photographed leaping completely out of the water by a full
body-length against a backdrop of the hazy, purple mountains of the Sea of
Cortez. A legendary American shark fisherman once characterized the Common
Thresher as “exceedingly stubborn” and, comparing this species with the
Shortfin Mako, states that it is “pound for pound, a harder fish to whip”.

The Common Thresher’s strength and endurance result from some fascinating
physiological adaptations. Like the Shortfin Mako and a few closely related
sharks, the Thresher has a strip of red muscle along each of its flanks.
This red muscle is highly aerobic, able to use oxygen efficiently so that it
can contract powerfully for long periods. Further, this red muscle features
a tight meshwork of tiny blood vessels that transfer metabolic heat so that
it can be shunted inward, toward the body core. Retaining body heat helps
the Common Thresher’s muscles function efficiently even in very cool water.

Their ability to retain body heat may explain the extraordinary range of
depths over which Common Threshers have been caught — from the surface all
the way down to at least 1,200 feet (365 metres). Often found close inshore
and in shallow bays, young Common Threshers seem to have difficulty
tolerating very cold or deep water. This may be because they are more
slender with proportionately longer upper caudal lobes than adults.
Therefore, young Threshers may radiate body heat to the surrounding water
faster than they can produce it.